<p>You’ve made it to the end of the ninth part of this series, so let’s summarize:</p>
<ol>
<li>Protocols describe what methods and properties a conforming type must have, but don’t provide the implementations of those methods.</li>
<li>You can build protocols on top of other protocols, similar to classes.</li>
<li>Extensions let you add methods and computed properties to specific types such as <code>Int</code>.</li>
<li>Protocol extensions let you add methods and computed properties to protocols.</li>
<li>Protocol-oriented programming is the practice of designing your app architecture as a series of protocols, then using protocol extensions to provide default method implementations.</li>
</ol>